Europe's elite clubs are suing Fifa over the issue of the release of players for international duty.
Fifa president Sepp Blatter has cut off contact with the G14 group
The G14 group wants Fifa to provide insurance and compensation to clubs for players who are injured while representing their countries.
It says Fifa's regulations are an abuse of the body's dominant position.
And G14 has backed Charleroi's fight for financial help after Abdelmajid Oulmers was sidelined for eight months when hurt playing for Morocco.
Oulmers tore ankle ligaments in a friendly against Burkina Faso last November and was subsequently out of action for the rest of the campaign.
Charleroi paid for Oulmers' surgery and his salary even though he was unable to play.
Fifa refuses to recognise or negotiate with the G14 group.
The case officially opened in the Charleroi Commercial Court on Monday.
G14 argues that Fifa's refusal to contribute towards players' salaries - even though clubs lose money when they suffer injuries in international matches - is illegal.
"As it is G14's wish to seek a once and for all clarification if these regulations are legal, it is sensible for us to join this case," said G14 general manager Thomas Kurth.
English clubs Manchester United, Liverpool and Arsenal are members of the G14 group, which has 18 members.